1. Introduction
This invention relates to light-sensitive compositions and more particularly, to light-sensitive compositions useful as solder resists in the manufacture of printed circuit boards.
2. Description of the Prior Art
For years, screen printing was the means used to apply a soldermask to a substrate in the manufacture of printed circuit boards. It was a desired process because it permitted selection of masking materials from a variety of resins in the formulation of the soldermask and enabled one to apply a heavy coating on an uneven surface with a reasonable degree of accuracy. However, increased packing of conductor tracks and reduced track widths resulted in a demand for greater dimensional accuracy and resolution of features beyond the resolution capability of conventional screen printing masks.
The accuracy and fine line definition capability of photoimagable systems has long been known, but photoimagable systems was initially not applied to the soldermask problem to obtain improved accuracy and line resolution.
A first step towards the formulation of a photoimagable soldermask was the introduction of screen printed U.V. photopolymerizable soldermasks utilizing acrylate resins. However, such photopolymerizable soldermasks did not gain significant market share.
A next step in the evolution of soldermasks was the introduction of the photoimagable soldermask analogous to the photoimagable photoresists used in printed circuit board manufacture to define a desired conductor pattern. Two primary methods were used to apply photoimagable soldermasks to a circuit substrate. One was as a dry film soldermask using a laminator to apply heat and pressure to the soldermask as it was applied to the circuit board substrate. The other method used a liquid soldermask curtain coated onto a substrate. Both dry film soldermask and curtain coated soldermask had advantages and disadvantages. Disadvantages common to both include use of organic solvent development, poor to moderate adhesion to copper and an industry desire for an increased resistance to elevated temperatures. The use of organic solvents presents environmental problems. Poor adhesion to copper results in delamination of the mask from the copper conductors during the soldering operation. Inadequate thermal resistance results in deformation of the mask during soldering.
More recently, aqueous developable liquid soldermask compositions have been developed in an effort to avoid the use of organic solvent developers. These soldermasks are generally applied as a wet coating, such as by curtain coating, dried, exposed, partially cured using heat and developed, typically with an aqueous alkaline solution and then fully cured by baking. An example of such a soldermask is disclosed in European patent publication No. 0,255,989. The soldermask of the published application consists of at least one solid film-forming phenolic resin, a compound that contains at least two epoxy groups or two vinyl groups or one of each in the molecule and a cationic photoinitiator such as a sulfonium salt.
As is known in the art, a soldermask must possess certain properties. It must adhere firmly to the underlying substrate, must resist degradation at soldering temperature, must have resistance to processing chemicals and must possess suitable dielectric properties. Moreover, if the soldermask is to be a photoimagable soldermask, the system must provide adequate image resolution.
Screen soldermasks were readily formulated to provide soldermasks with suitable properties because resin formulations could be prepared that were not required to be compatible with imaging chemicals. The selection of resins for formation of photoimagable soldermasks was curtailed because many resins used in soldermask formulations are not suitable in a photoimagable system. Provision for an aqueous developable soldermask further curtailed the availability of resins because portions of the soldermask must be soluble in an aqueous developer.
Where selection of resins for an aqueous developable soldermask is limited, difficulties have been encountered in efforts to provide an aqueous developable soldermask coating having properties comparable to the properties of a screened soldermask. In particular, dried coatings formed from aqueous developable soldermasks have been found to exhibit reduced adhesion to copper at elevated temperatures, poor thermal properties and poor resistance to processing chemicals such as fluxes.